1. Technical Field
The present invention relates to a diaphragm for a pressure sensor and to a pressure sensor having the diaphragm. The invention is particularly suitable for suppression of aging deterioration of the diaphragm.
2. Related Art
Pressure sensors that use a piezoelectric resonator as a pressure sensitive element are known as a water pressure gauge, an air gauge, and a differential pressure gauge. The piezoelectric resonator includes, for instance, an electrode pattern on a plate-like piezoelectric substrate, and a direction in which a force is detected is set to be a detecting axis. When pressure is applied in the direction of the detecting axis, a resonance frequency of the piezoelectric resonator changes, and the pressure is detected by using the changes in the resonance frequency. JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, as a first, second, and third examples, disclose a pressure sensor including a piezoelectric resonator as a pressure sensitive element. When a pressure is applied to bellows from a pressure input orifice, a force F corresponding to an effective area of the bellows is transmitted to the piezoelectric resonator as a compressive force or a tensile force through a force transmitting unit that has a pivot as a fulcrum (a flexible hinge). A stress corresponding to the force F is generated in the piezoelectric resonator, and this stress changes the resonance frequency. The pressure sensor measures the pressure by detecting changes in the resonance frequency of the piezoelectric resonator.
A pressure sensor of related art will be described with reference to the first example and the like. FIG. 10 is a schematic view showing a structure of the related art pressure sensor.
With reference to FIG. 10, a pressure sensor 501 according to the related art includes a case 504 having first and second pressure input orifices 502 and 503 that are arranged to face each other, and a force transmitting member 505 disposed inside the case 504. One end of the force transmitting member 505 is sandwiched between and coupled to one end of a first bellows 506 and one end of a second bellows 507. The other end of the first bellows 506 is coupled to the first pressure input orifice 502, and the other end of the second bellows 507 is coupled to the second pressure input orifice 503. Moreover, a double-ended tuning fork resonator 509 serving as a pressure sensitive element is disposed between the other end of the force transmitting member 505 and an end of a substrate 508 which is an opposite end from a pivot (fulcrum).
The bellows of this pressure sensor is filled with a liquid so as to detect pressure with high precision. Generally, an oil such as silicon oil having high viscosity is used as the liquid in order to prevent bubbles from entering and accumulating inside the bellows or between the folds of the bellows.
Thus, the interior of the first bellows 506 is filled with oil 510 having viscosity. When the object of pressure measurement is a liquid, the oil 510 faces and contacts the liquid at an opening 511 opened at the first pressure input orifice 502. The size of the opening 511 is set such that the oil 510 does not leak out.
In the pressure sensor 501 having such a structure, a pressure F is applied from the liquid subjected to pressure measurement to the oil 510 filling the first bellows 506. The pressure F is then applied to the one end of the force transmitting member 505 (a pivotably supported swing arm) through the first bellows 506. At the same time, atmospheric pressure is applied to the second bellows 507, and a force equivalent to the atmospheric pressure is applied to the one end of the force transmitting member 506.
Consequently, a force equivalent to a differential pressure is applied through the other end of the force transmitting member 505 to the double-ended tuning fork resonator 509 as a compressive force or a tensile force, using the pivot of the substrate 508 as a pivoting point. The differential pressure here means a pressure difference between the atmospheric pressure and the pressure F applied by the liquid that is subjected to pressure measurement. When the compressive force or the tensile force is applied to the double-ended tuning fork resonator 509, a stress is generated in the resonator 509. In accordance with the strength of the stress, the resonance frequency of the resonator 509 changes. Therefore, the strength of the pressure F is detected by measuring the resonance frequency.
JP-A-2005-121628 as a fourth example discloses a sensor that does not include an expensive force transmitting unit (a cantilever), which is used in the pressure sensor mentioned above, having a swing arm with the pivot (the flexible hinge) used as the fulcrum. In this sensor, two bellows are collinearly aligned in a sensor housing while sandwiching a pedestal therebetween. From a behavior of the pedestal in an expansion and contraction direction of the bellows, the sensor detects pressure fluctuation that is caused by the difference between pressures introduced to each of the bellows. The pedestal for bonding the resonator is therefore sandwiched between one end of the first bellows and one end of the second bellows, and both ends of a pressure sensitive element provided at a circumference side of the second bellows are fixed respectively on the pedestal and on a housing wall on a side adjacent to the other end of the second bellows. Additionally, a reinforcing plate is disposed axisymmetrically to the pressure sensitive element, with the second bellows interposed therebetween. Both ends of the reinforcing plate are fixed on the pedestal and on the housing wall.
JP-A-2007-57395 as a fifth example discloses a pressure sensor including a reinforcing flexible member (namely, a string) that connects a pedestal to a housing and is disposed in a direction orthogonal to a direction of a pressure-detecting axis. The reinforcing flexible member is provided so as to solve a problem that the sensor disclosed in the fourth example has an insufficient strength against a shock coming from a direction orthogonal to a direction of a pressure-detecting axis of the bellows.
JP-A-2006-194736 and JP-A-2007-132697 as sixth and seventh examples disclose a pressure sensor that is used in a fixed manner to an engine block so as to detect hydraulic pressure inside an engine. This pressure sensor includes: a sensing unit that outputs an electric signal corresponding to an applied pressure, a pressure receiving diaphragm unit that receives pressure, and a pressure transmitting member that transmits the pressure from the diaphragm unit to the sensing unit. Specifically, a first diaphragm for receiving pressure is installed on one end surface of a hollow metal stem, and a second diaphragm for detection is installed on the other end surface of the hollow metal stem. The pressure transmitting member is provided between the first and second diaphragms in the stem. The pressure transmitting member is a shaft made of metal or ceramic, and is provided between the pair of diaphragms in a prestressed state. Moreover, a chip with a function of a strain gauge as a pressure detection element is attached to an outer end surface of the second diaphragm. The pressure transmitting member transmits pressure received by the first diaphragm to the second diaphragm, and deformation of the second diaphragm is converted into an electronic signal by the strain gauge chip, thereby detecting the hydraulic pressure of the engine.
JP-A-2005-106527 and JP-A-2005-106528 as eighth and ninth examples disclose a pressure sensor which includes a diaphragm that seals a pressure input orifice of a housing containing a pressure sensitive element. A structure disclosed in these examples is such that the diaphragm is weld-attached by laser welding or electron beam welding. When welding, a ring-shaped bead (a welded and solidified portion) is formed at a portion where the diaphragm is welded to the housing.
In the pressure sensor of the first to third examples as shown in FIG. 10, the first bellows 506 is filled with the oil 510. However, the oil 510 has a higher thermal expansion coefficient compared to those of other components constituting the pressure sensor 502 such as the force transmitting member 505 and the double-ended tuning fork resonator 509. Therefore, these components may become thermally deformed due to a temperature change. Such thermal deformation is an unwanted stress to the double-ended tuning fork resonator 509 and induces errors in measurement of pressure values. Thus, the characteristics of the pressure sensor are degraded.
Moreover, since the oil 510 filling the first bellows 506 contacts and faces the liquid subjected to the pressure measurement, the oil 510 may flow into the liquid, or the liquid may flow into the first bellows 506, depending on how the pressure sensor is installed. This may generate bubbles inside the oil 510 filling the first bellows 506. If bubbles are generated in the oil 510 that serves as a pressure transmitting medium, a force cannot be stably transmitted through the force transmitting member 505 to the double-ended tuning fork resonator, thereby possibly inducing errors in the pressure value measurement.
Also, as described above, since the oil 510 contacts and faces the liquid subjected to pressure measurement, the oil 510 may flow into the liquid depending on how the pressure sensor is installed. Therefore, the related art pressure sensor using the oil 510 is not applicable to measurement of pressure of a pure liquid that dislikes foreign substances.
Furthermore, it is difficult to miniaturize the pressure sensor 502 of the related art because it includes the force transmitting member 605 having a complicated structure. Also, because the force transmitting member 505 needs a flexible hinge having a slim constriction and is thus an expensive component, the manufacture of the pressure sensor becomes costly.
The pressure sensor of the fourth and fifth examples has a problem that, when the pressure sensor is inclined, the bellows thereof droops. The force applied to the pressure sensitive element (the double-ended tuning fork resonator) therefore changes, leading to fluctuation of the resonance frequency.
The structure of this pressure sensor is such that one end of a pipe filled with oil is coupled to a pressure introduction orifice of the pressure sensor, and that the other end of the pipe is in contact with the liquid to be measured. Therefore, as is the case with the first to third examples, the oil filling the bellows or the pipe contacts and faces the liquid subjected to pressure measurement. Accordingly, the oil may flow into the liquid subjected to pressure measurement or the liquid may flow to the bellows, depending on how the pressure sensor is installed. Therefore, bubbles may be generated in the oil filling the bellows. If bubbles are generated in the oil, the oil serving as a pressure transmitting medium does not stably transmit the pressure through the pedestal to the double-ended tuning fork resonator, resulting in errors in the pressure measurement.
In the pressure sensor of the fifth example, the pedestal sandwiched between the bellows is supported by the reinforcing flexible member that is a plate spring provided at a lateral surface of the housing. Therefore, a suppressing force is likely to affect the pedestal's behavior that is accompanied by the movement of the bellows in the axis direction. Therefore, pressure detecting sensitivity may be decreased. Also, if the reinforcing flexible member is hardened in order to strengthen its support, the movement of the bellows is suppressed, thereby decreasing the pressure detecting sensitivity.
Furthermore, in the fourth and fifth examples, the reinforcing plate is disposed axisymmetrically to the pressure sensitive element, with the bellows interposed therebetween. Therefore, the movement of the bellows is suppressed, thereby decreasing the pressure detecting sensitivity.
In the sixth and seventh examples, the diaphragm and the shaft are in contact with each other in the prestressed state, and the pressure sensor is used at high temperature and pressure. Therefore, if the diaphragm and the shaft are rigidly fixed, the mechanism may be damaged by thermal expansion that is different among the components. The diaphragm and the shaft have only a point contact in order to avoid the thermal expansion and are not bonded by a bonding means such as an adhesive. Therefore, there is a very high possibility that this contact point deviates when the diaphragm and the shaft operate due to the pressure changes. As the contact point deviates, a force acting on both the diaphragm and the shaft leaks out, resulting in less accurate pressure detection. Moreover, because the pressure sensor of the sixth and seventh examples is used at high temperature and pressure, it is desirable that the force transmitting member be as long as possible in order to create a distance between the pressure receiving unit and the sensing unit and to avoid thermal influence on the components such as the chip of the sensing unit. Accordingly, the sensor of these examples is not suitable for miniaturization. In addition, in the sixth and seventh examples, a force is transmitted through the shaft disposed between the pair of diaphragms. However, since a sensor chip is mounted on one of the diaphragms on a sensing unit side, the property of the diaphragms differs between the pressure receiving side and the sensing unit side. Therefore, the measurement accuracy may not be increased.
Furthermore, the pressure sensor in the eighth and ninth examples is such that, when the laser or electron beam irradiation is stopped, the bead becomes thermally deformed due to rapid cooling and is therefore highly susceptible to cracks and the like. If a pressure sensor having such a diaphragm is used, the bead becomes repeatedly deformed because of recurring deformation of the diaphragm that receives pressures from outside. This makes the cracks to grow, leading to problems such as aging deterioration and damages in the diaphragm.
Additionally, the diaphragm that is thermally expanded by the heat from the laser irradiation contracts as it cools down after the laser irradiation is stopped. A residual stress generated at this time is concentrated on a central area of a pressure receiving part of the diaphragm, and the central area is nonuniformly deformed, leading to decrease in the sensitivity of the pressure sensor.
Furthermore, since the welding portion welds at a high welding temperature, it is exposed to high temperature in the laser irradiation and is therefore susceptible to brittle fracture.